1. Field of the Invention
The present invention relates to a communication system suited for being applied to a case of, for example, receiving various kinds of broadcasting signals such as a digital satellite broadcasting, a digital ground broadcasting and so on or equivalent communication signals.
2. Description of the Related Art
In the past, the transmission side of a digital broadcast for carrying out a television broadcast, a radio broadcast and the like by transmission of digital data has been carried out by an arrangement shown in FIG. 1. Here, a broadcasting center 1 has such an arrangement to carry out both transmission of a digital satellite broadcast and transmission of a digital ground wave broadcast, and carries out coding processes of respectively corresponding data of a video source 1a supplied by a contents provider and the like in a video coding unit 1c, an audio coding unit 1d, an accessory data unit 1e in a source coding unit 1b and multiplexing process of respectively coded data in a multiplexing unit if. Here, as for the coding process, video data and audio data are subjected to a coding process by, for example, a MPEG-2 system (one of standardized coding systems of moving images) (concretely, for example, video data by a process prescribed by MPEG-VIDEO ISO/IEC 13818-2 and audio data by a process prescribed by MPEG-2 AUDIO ISO/IEC 13818-3 or the like) and as for an accessory data, arbitrary coding is carried out. The multiplexing process in the multiplexing unit if is carried out in accordance with a predetermined system, for example, MPEG-2 System (ISO/IEC 13818-3). When subjected to the MPEG-2 System, the data is converted into a transport stream (hereafter, referred to as a TS) of this MPEG-2 System. The following explanation will be made on condition that the data is processed by this MPEG-2 System.
The coded signal (TS) in the source coding unit 1b is supplied to a channel coding unit 1g to be converted into a signal format to be practically transmitted. That is, a scramble process is carried out for energy diffusion in a scramble processing unit 1h, and in an error correcting coding unit 1i, generating and adding of an error correcting code are carried out. As for the error correcting coding here, a continuous code combining a blockcode as an outside signal and a convolution signal as an inside code are used. As the outside signal, for example, a lead solomon code and as the inside code, for example, a punctured code with a variable coding ratio are used. After the error correcting coding process is carried out, an interleave process is carried out in an interleave unit 1j. 
Then, in a case of a ground wave broadcast, a modulating process of an output from the channel coding unit 1g is carried out by an OFDM system (Orthogonal Frequency Division Multiplex system) in an OFDM modulating unit 1k and a conversion process of its modulated signal as a radio signal with a predetermined transmission band area is carried out in a RF conversion unit in and radio transmission is carried out from a ground wave antenna 2. Also, in a case of a satellite broadcast, a modulating process of an output from the channel coding unit 1g is carried out in a QPSK modulating unit lm with a QPSK system (Quartenary Phase-Shift keying System) and a conversion process of its modulated signal as a radio signal with a predetermined transmission band is carried out in a RF modulating unit 1o and radio transmission is carried out toward a broadcasting satellite (or a communication satellite) by a satellite antenna 3.
Then, as an arrangement on the reception side of a digital broadcasting signal transmitted with this arrangement, there is used a conventional arrangement as shown in FIG. 2. A ground wave reception antenna 4 and a satellite reception antenna 7 for receiving a broadcasting wave from a broadcasting satellite 6 are connected to a set-top box 5 which carries out a reception processing for a broadcasting signal and the ground wave reception antenna 4 is connected to a ground wave tuner 5a, and a signal received by the tuner 5a is demodulated in an OFDM demodulation unit 5b to obtain reception data. Also, the satellite reception antenna 7 is connected to a satellite tuner 5c and a signal received in this tuner 5c is demodulated in a QPSK demodulation unit 5d to obtain reception data.
Digital broadcasting data demodulated at respective demodulating units 5b and 5d are subjected to a deinterleave process in a deinterleave unit 5e, an error correcting process in an error correcting unit 5f and a descrambling process in a descramble unit 5g, whereby the original TS is restored. Then, video data and audio data with MPEG-2 system are subjected to a separation process in a multiplex separation unit 5h and decoded in a MPEG 2 decoder 5i. The video data decoded in the decoder 5i is made to be a video signal of NTSC system in an NTSC encoder 5j and the audio data decoded in the decoder 5i is made to be an analog audio signal in a digital/analog converter 5k, and these video signal and audio signal are supplied to and received by a monitor receiver 8 (or an ordinary television receiver) connected to the set top box 5.
By connecting an apparatus called the set top box to a receiver in this manner, it becomes possible to watch the digital broadcast by a receiver corresponding to an analog ground wave.
Here, the arrangement has been shown of transmission and reception of the digital broadcast by the ground broadcasting wave and the satellite broadcasting wave, but in a case of a cable television (so-called CATV), the arrangement is the same until the channel coding on the transmission side and is different only in that a particular arrangement is used in the modulation process and the RF conversion process.
On the other hand, as an entirely different system from a watching system for the television broadcast, what they call a multimedia mobile access system (MMAC: Multimedia Mobile Access System) has been proposed. This access system is a high speed radio access system capable of being seamlessly connected to an optical fiber network (BISDN), in which a comparatively high frequency band such as 5 GHz and the like is used as a frequency band, a transmission rate is about 30 Mbps and a TDMA/TDD system (time division multiple access system) is used as an access system. FIG. 3 is a diagram showing an overall arrangement of this multimedia mobile access system and here is an arrangement in a case of carrying out a service called an IP (Internet Protocol) access connected to an Internet network, and which comprises a various kinds of contents server 11 connected to an Internet network 12 and an MMAC base station 15 where communication is carried out through an ISDN (or general telephone line) 13 or an optical fiber network 14. This base station 15 is connected to the ISDN 13 or the optical fiber network 14 by a user network interface (UNI).
The MMAC base station 15 carries out a radio communication with a portable information terminal 16 according to the above-mentioned transmission system and a relay between circuits 13 and 14 connected to the base station 15 and the terminal 16.
FIG. 4 shows an arrangement of the MMAC base station which has conventionally been proposed, and here is an example of a case when the optical fiber network 14, where communication is carried out by an asynchronous transfer mode (Asynchronous Transfer Mode: hereafter, referred to as an ATM), is connected. In the base station 15, an interface unit 15a which carries out a user network interface with data (ATM cell) transmitted in terms of the ATM is connected to the optical fiber network 14 to carry out multiplexing of the ATM cell. In an ATM network circuit control unit 15b connected to an interface unit 15a, circuit control such as a call access and the like on the network is carried out. In an ATM cell diassembly/assembly unit 15c connected to the ATM network circuit control unit 15b, diassembling of the ATM cell from the network side and assembling of the ATM cell transmitted to the network side are carried out.
Received data from the network side, which is diassembled in the ATM cell diassembly/assembly unit 15c is sent to a MMAC channel coding/decoding unit 15d to be converted into a MMAC radio transmission format, and after this converted data is subjected to a modulation process by way of the QPSK modulation or the like in a modulation unit 15g, transmission processes such as frequency conversion, amplification and the like are A carried out in a transmission unit 15h and radio transmission from an antenna 15i to a terminal is carried out.
Also, a signal transmitted from the terminal side is such that, after being subjected to a reception process such as frequency conversion and the like in a reception unit 15j connected to the antenna 15i, demodulation of received data is carried out in a demodulation unit 15k, and the demodulated received data is supplied to the MMAC channel coding/decoding unit 15d for a decoding process. Then, the decoded data is assembled as an ATM cell in the ATM cell diassembly/assembly unit 15c and transmitted from the interface unit 15a to the optical fiber network 14 connected under control of the ATM network circuit control unit 15b. 
Also, these processes in the MMAC base station 15 are, carried out under control of a central control unit (CPU) 15e through a bus line 15f. 
In the portable information terminal 16 which is a MMAC terminal, as shown in FIG. 5, after reception processes such as frequency conversion and the like are carried out in a reception unit 16b connected to an antenna 16a, demodulation of the received data is carried out in a demodulation unit 16c, the demodulated received data is supplied to an MMAC channel coding/decoding unit 16d to carry out a conversion process from a radio transmission format of the MMAC. This converted data is, after being supplied to a central control unit (CPU) 16g of the terminal 16, separated into video data and audio data, which are supplied to a digital signal process unit (DSP) 16k to be subjected to a decode process based on the MPEG-2 system. After the video data is processed for a display, it is supplied to a liquid driver 16i and a video is displayed on a liquid display 16j based on control of the central control unit 16g. Also, the audio data included in the received data is converted into an analog audio signal in the digital signal process unit 16k and is outputted from a speaker 16m. 
Also, transmission data generated based on operation by an operation unit 16h connected to the central control unit 16g is supplied to the MMAC channel coding/decoding unit 16d to be converted into the radio transmission format of the MMAC, and after this converted data is subjected to a modulation process by way of the QPSK modulation or the like in a data modulation unit 16e, the modulated data is subjected to transmission processes such as frequency conversion, amplification and the like in a transmission unit 16f and radio-transmitted from the antenna 16a to the base station.
By preparing a base station and a terminal unit with such a MMAC system and being connected with the Internet network or the like, it is possible to receive an Internet broadcast from the various kinds of contents server or the like by the terminal unit 16. In this case, in a case of the MMAC system, because a high speed radio access is possible, it is possible to have the terminal unit receive and display moving video data and the like.
By the way, the MMAC system terminal unit can carry out the reception and transmission as it moves as long as it is within a range where a signal from the base station can reach as well as it can be made to receive and display the Internet broadcast or the like at an arbitrary place, but, it has been difficult to incorporate a digital broadcast receiving system shown in FIG. 2 in the MMAC terminal unit.
That is, as for an antenna for receiving a digital broadcast, particularly, a digital satellite broadcast, there is a need for an antenna having a high-gain as well as high directivity and further, because there is a need that a beam is oriented toward a constant direction, it is difficult for the portable terminal to carry out reception. Also, there is a limit to miniaturizing a receiving process apparatus for the digital broadcast called the set top box 5 shown in FIG. 2, thereby making it impossible to install it in the portable terminal. Particularly, in a case of an apparatus for carrying out reception from both the satellite broadcast and the ground wave broadcast as shown in FIG. 2, a circuit becomes necessary to carry out receiving process of each of the signals, thereby posing a cause for hampering the miniaturization and low cost of the terminal unit. Also, as a matter of course, it has been impossible for a mobile terminal to receive a digital broadcast transmitted by cable television.
Also, it may be conceivable that the Internet broadcast and large capacity contents are Internet-accessed and distributed by mobile communication according to the MMAC system but, for many users to try a simultaneous access, there has been a need for a network backbone with high speed and wide band as well as an enormous burden on the part of servers, with a result that there has been a limit to distributing high speed and large capacity data only with the present ground-system network.
In view of such points, an object of the present invention is to carry out satisfactory reception of various kinds of data such as the digital broadcast, the Internet broadcast and the like by a mobile terminal.
According to a first aspect of the present invention, there is provided communication system, which comprises
a relay apparatus having a receiving unit having a plurality of receives for receiving communication signals communicated by a plurality of different communication systems,
a control unit for selecting one of said plurality of receivers based on a request signal,
a converting means for converting a communication signal, which is received by a receiver selected by said control unit, into a radio signal corresponding to a specific radio transmission format, and
a radio transmission unit for transmitting said radio signal; and
a terminal apparatus having a receiving means for receiving a radio signal corresponding to said specific radio transmission format.
According to a second aspect of the present invention, there is provided a relay apparatus in communication system, which comprises
a receiving unit having a plurality of receivers for receiving communication signals communicated by a plurality of different communication systems;
a control unit for selecting one of said plurality of receivers based on a request signal;
a converting means for converting a communication signal, which is received by a receiver selected by said control unit, into a radio signal corresponding to a specific radio transmission format; and
a radio transmission unit for transmitting said radio signal to a terminal apparatus corresponding to said specific radio transmission format.
According to a third aspect of the present invention, there is provided a terminal apparatus capable of communicating with a relay apparatus having a receiving unit having a plurality of receivers for receiving communication signals communicated by a plurality of different communication systems,
a control unit for selecting one of said plurality of receivers based on a request signal,
a converting means for converting a communication signal, which is received by a receiver selected by said control unit, into a radio signal corresponding to a specific radio transmission format, and
a radio transmission unit for transmitting said radio signal, which comprises a request signal generating means for generating said request signal; and
a request signal transmission means for transmitting said request signal; and
a receiving means for receiving a radio signal corresponding to said specific radio transmission format.